Fluff, dust, lint, micro-fibrils and stickies combine together and make a composite, posing biggest technical challenges in using recycled fiber in papermaking. Fluff and stickies cause product quality issues, such as holes and specks in paper as well as paper and rewinder breaks, higher paper rejection or downtime. It is common to have a build-up of black and sticky or gummy deposits on the wire, felts and dryer fabrics. Fluff or lint along with stickies at dryer section has become a serious threat as quality of recycled raw material degrades day by day owing to over recycling.
While producing packaging grade paper from recycled paper furnish e.g. old corrugated containers (OCC), old newsprint (ONP), mixed office waste (MOVV), household waste, often machine runnability and rewinder runnability may be seriously affected due to fluff or lint issue at the dryer section along with stickies. Fluff, dust, lint or fiber and stickies combine together to make a composite, posing one of the biggest challenge in using recycled fiber in papermaking. Fluff and stickies cause product quality issues, such as holes and specks in paper apart from runnabillity issues, even at customers end, leading to higher paper rejection or downtime. It is common to have a build-up of black and sticky or gummy deposits on the wire, felts and dryer fabric. Fluff or lint along with stickies at dryer section has become a serious issue as quality of recycled raw materials is degrading day by day. While doctoring the cylinders became essential to keep the cylinder surface clean but accumulation of fluff and stickies or composite of these two at doctor blades become a serious problem. Frequent machine stoppage due to washing of fabrics and cleaning of doctor blades becomes necessary to maintain good runnabillity at various stages of paper making process and usage e.g at paper machine, winder, converting machine, box making machine etc. All of these problems cause lost production time and mounts a higher cost for the papermaker and converter. It is often difficult to estimate the exact loss due to stickies deposit, since it is often not certain if only the stickies caused the break. This analysis gets more complex as most of the time, stickies sample reveals its composite nature. Typically, a sticky sample could consist of cellulosic materials, extractable organic substances and inorganic components e.g. talc, calcium carbonate, silicate etc.
It is therefore relevant to understand stickies and fluff or lint issues independently and then find an innovative solution for this compound and complex problem called ‘Sticky’.
As known to a person skilled in the art, lint is defined as poorly bonded surface material from the surface of the paper sheet. Loosely bonded materials could be fibre, fines, ray cells, fillers, vessels etc.
Lint control strategies can be broadly categorized into three. One commonly used practice being reducing dryer temperatures in the first few sections and ensuring clean smooth dryer surfaces to minimize surface disruptions and hence dusting. Increased press loadings help to reduce dusting. Followed by chemical route being addition of retention aids and other lint control additives to improve retention and thereby control lint or dusting. Internal sizing, Starch addition to the wet end, defoamers and more recently specialized roll release aid chemicals, which contains hydrocarbon materials etc. reduce surface disruption. However, one of the most effective strategies for lint control is to select furnish and develop the fiber in a desired manner with improved fines distribution.
As known to a person skilled in the art, another factor contributing fluff and stickies issue had been water closure in paper mills. This leads to fines build up in the system resulting fluff or lint issue at initial dryers of the drying system. On top of it, stickies issue get compounded as some mills put back recovered fiber/sludge from white water system or mill effluent water treatment plant into pulper or pulp system to achieve high yield.
Generally, the term “stickies” refers to sticky material in the recovered paper but excludes wood extractives. The most common sources of stickies are adhesives used in attaching labels, advertisements, CD's and any other additional material in newspapers and magazines, for binding catalogues, for envelope sealing, stamps, and many more; the other sources of stickies are ink binders and coating binders, i.e. latex. Wax used in carton for packaging is also a significant source of stickies in recycling of old corrugated containers (OCC).
As stated in US Patent 2006/0048908A1, Stickies are classified in two groups. Various adhesives like styrene-butadiene copolymer (SBR), polybutradiene (PBD), polyethylene (PE), and polypropylene (PP); hot melts such as poly-vinyl acetate (PVAc), waxes, resins, vinyl acrylates. Ink residues like resin esters, mineral oils, alkyd resins, styrene-acrylates, and epoxy acrylates; and wet-strength chemicals like urea-formaldehyde considered as synthetic type of stickies.
Natural substances, which contribute to stickies, include fatty acids or esters such as resin acids, wood resins, and, fatty acids, fatty salts, etc. Substances classified as wood extractives may also include, in addition to actual wood resin, fatty acids from deinking aids or resin acids from rosin sizes.
Physically, stickies may be divided into macro, micro and secondary stickies. Macro stickies refer to tacky particles that retain on a laboratory screen of 100 or 150 μm (Blanco et al. 2002; Doshi et al. 2003a; Hamann & Strauss 2002). The size comes from the usual slot size of fine screening. Stickies smaller than 100 or 150 μm, but bigger than 1-5 μm (Doshi et al. 2003a; Hamann & Strauss 2002) are called micro stickies. Dissolved and colloidal stickies smaller than micro stickies are called secondary or potential secondary stickies (Faul 2002; Hamann 2005a). The secondary stickies are thought to become harmful after changes in temperature, pH, or chemical environment (Blanco et al. 2002; Carré et al. 1998; Li & Zhan 2005)
Mills adopt various methods to control stickies and fluff having independent effects or combined effect. As stated in Patent WO 20060294004A1, one of the methods to control stickies is to use expensive wastepaper grades that contain less stickies, however availability of such waste material is scarce and costly. Other methods described for control of stickies are by mechanical means, include cleaning, screening, and dispersion, flotation, washing.
As stated in Patent WO 20060294004A1 the larger or macro stickies are removed by Screening. Centri-cleaners help remove stickies on density differences using centrifugal force. Heavier stickies can be removed by Forward cleaners, whereas lighter particles are removed by reverse cleaners. Flotation helps to remove stickies from back-water or white-water which are intermediate in size stickies. Sludge generated from Flotation Cell contains significant amount of stickies and inorganic substance. Another commonly adopted method to deal with stickies is Dispersion. Pulp is thickened, passed through rotating equipment at high temperature, pressure, and shear, result in breakdowns organic contaminants, including stickies, into smaller pieces.
The fibers that of context for this invention are cellulose fibers and more typically recycled fibers from a variety of paper products or fiber containing products, such as old corrugated containers (OCC), old newsprint (ONP), mixed office waste (MOVV), household waste (HW), other recovered paper varieties, or combinations thereof. Recycled fibers are not as strong as virgin fibers due to a loss of hydratability, swelling and fiber flexibility that occurs in the paper forming and drying process.
It was relevant to understand that the main difference between once-dried (secondary) fiber and never-dried (virgin) fiber is that dried fibers are hornified; that is, irreversible fiber bonding within the fiber wall has occurred that resists re-swelling. The drying process has been reported to decrease the specific volume and surface areas of pulp. The dried fibers are difficult to wet because of their glassy crystalline nature compared with the highly amorphous, hydrophilic virgin fiber. In addition, the redistribution of olefinic compounds during drying results in self-sizing. The increased crystallinity of dried fiber also renders the fibers more brittle and difficult to bond leads to dust or lint or fluff generation at pre-dryer section. Hornification is traditionally measured by determining the reduction in the water retention value (WRV) of the pulp after a cycle of drying, although WRV measurement cannot reveal exactly how the fiber has changed. Studies suggest that mechanical pulps resist hornification as evidence that hemicelluloses and possibly lignin inhibit hornification impact. It is indicated, as the Hemicelluloses and lignin on the cellulose fiber surface prevent intrafiber cellulose interaction, by blocking the functional groups on adjacent cellulose surface, the potential for hornification is reduced, and the rewet fiber retains a greater percentage of the available bonding strength it possessed before the initial drying. However, while hemicelluloses can aid in the development of paper strength, lignin inhibits interfiber bond formation, and the inventors brought in a enzyme component in the mix to address the Lignin part (Brancato 2008).
To deal with stickies issues, usual chemical routes include are fixation, detackification etc. Adopting fixation methods, the stickies are attached to the paper sheet by using a suitable polymer. In detackification process, additives like talc, clay, nonionic organic polymers, other inorganic particles, and enzymes are used to convert stickies less tacky in nature.
Other practical approaches being adopted to control fluff and stickies issues are purging white water from selective locations. Avoid using sludge/recovered fiber having lot of contaminations in certain grades of papermaking. Fractionation of pulp and use suitable fraction of the pulp at appropriate layer.
Unfortunately, even with the best of all of these methods, too many of the stickies are still present in the final product to avoid problems.
It is therefore an object of the present invention to reduce rewinder breaks resulting from stickies and their build-up supported by fluff, lints and micro-fibrils.